The present invention generally relates to impairment detection and compensation in a communications network, and more particularly to a method and apparatus for detecting and compensating digital losses in a communications network.
A modem converts binary data into modulated analog signals, which are typically transmitted over facilities originally designed for voice transmissions, such as the public switched telephone network (PSTN). Conventional modems treat the PSTN as a pure analog channel even though the signals are in digital form during most of transmission, while pulse code modulation (PCM) modems take advantage of the fact that most of the network is digital and central site modems (e.g., internet service providers and on-line services) are connected to the PSTN via digital connections (e.g., a North American T1 or a European E1 digital connection).
Central site PCM modems transmit eight bit digital words, also referred to as octets or points, corresponding to different coder-decoder (CODEC) output levels. At a central office or digital loop carrier (DLC), the octets are converted to analog levels and transmitted over an analog loop to an end user modem, which maps the equalized digital levels to the originally transmitted octets. However, due to digital impairment in the digital network, including digital loss (DL) and/or robbed bit signaling (RBS), the octets may be corrupted during transmission. This corruption generally results in the generation of a modified level at the central office or DLC, which is received and processed by the end user modem, resulting in a mapping that may not accurately produce the equalized digital levels corresponding to the octets originally transmitted by the central site modem.
In order to mitigate the adverse affects of impairments introduced in the digital network, the impairments are identified during modem training modes. Generally, modem-training modes involve transmission of a training point sequence or Digital Impairment Learning (DIL) sequence between the central site modem and the end user modem, which is analyzed by the end user modem to detect the impairments or combinations of impairments in the communications network. These impairments or combinations of impairments identified by the end user modem are subsequently used to compensate for the digital impairments introduced during transmission. Therefore, methods and apparatus are desirable for detection and compensation of impairments introduced by the digital network of a communications network.